Rotor Analysis -- Big deformation problem

[Stef]

Hi everyone,
i am currently doing a rotor analysis with "modal analysis" on Ansys and even though my rotor's specs are L=145mm D=10mm and the rotating velocity is 10000rpm i get a deformation of 100mm at most of the modes. I have read that this might be cause of a mass-normalized value but i am not sure if that's the case. Does anyone know why is this happening and if so, can i change it so i can see the real value?
Thanks in advance.

 

[Dr.D]

Modal analysis only gives you the eigenvectors. The absolute magnitudes mean nothing at all, only the relative magnitudes are significant. I suspect that this is what you are seeing; I've seen it myself with FEA modal analysis.

 

[Stef]

Yes it looks like it. Maybe I have to use some other way. Thank you anyway.

 

[Dr.D]

You speak of wanting to see the "real value." The difficulty with that is that there is no "real value" until you specify the excitation. As long as you analyze an ideal, perfectly balanced rotor, there is not way to evaluate the vibration amplitude; it is indeterminate.

Consider a simple example to clarify this situation. Consider a mass M on a spring with constant K, with position described by x. The equation of motion is
M*DDx + K*x = 0
where D = d/dt.
As you know, the solution is x = A*cos(omega_n*t) + B*sin(omega_n*t)
where
omega_n^2 = K/M
But, and this is the point, A and B cannot be determined without initial or boundary conditions. The solution just described is called the transient (homogeneous) solution. You may wonder why it is called the transient solution when it clearly persists forever.

In the discussion above, damping was omitted, but in all real systems, damping is present. The inclusion of positive damping of any type (viscous, V^2, dry friction, hysteretic, etc) will cause this solution to disappear as the time becomes large.

Now, if we put an excitation term on the right side of the equation, the steady state (particular) solution will have a definite amplitude, but the homogeneous solution is still of undefined amplitude. In many cases, we simply say that enough time has elapsed to cause the transient to disappear.

 

[Stef]

That was very helpful thank you very much for your explanation. Have a nice day.